The present invention relates to three dimensional computerized tomography. More specifically, the present invention relates to completing or supplementing incomplete scanning path data from cone beam three dimensional (3D) computerized tomography (CT).
In conventional computerized tomography for both medical and industrial applications, an x-ray fan beam and a linear array detector are used. Two-dimensional (2D) imaging is achieved. While the data set may be complete and image quality is correspondingly high, only a single slice of an object is imaged at a time. When a 3D image is required, a stack of slices approach is employed. Acquiring a 3D data set one 2D slice at a time is inherently slow. Moreover, in medical applications, motion artifacts occur because adjacent slices are not imaged simultaneously. Also, dose utilization is less than optimal because the distance between slices is typically less than the x-ray collimator aperture, resulting in double exposure to many parts of the body. In 2D CT, the scanning path of the source is often simply circular scan about the object. The linear array detector is fixed relative to the source. (Although it is usual to talk about a scan path of a source relative to the object to be imaged, it is to be appreciated that the object may be rotated or otherwise moved to provide relative motion between the object and the source.)
In a system employing true cone beam geometry for 3D imaging, a cone beam x-ray source and a 2D area detector are used. An object is scanned, preferably over a 360.degree. angular range, either by moving the x-ray source in a scanning circle about the object or by rotating the object while the source remains stationary. In either case, the area detector is fixed relative to the source. The relative movement between the source and object which is to be imaged provides scanning in either case. Compared to the conventional 2D stack of slices approach to achieve 3D imaging, the cone beam geometry has the potential to achieve rapid 3D imaging of both medical and industrial objects with improved dose utilization.
The standard scanning path used in cone beam 3D CT imaging is a single circular scan of source and detector about the object. However, the data acquired in a single scanning circle can be shown to be incomplete for 3D CT imaging.
Complete 3D data scanning paths are known, but they are often complex. For example, U.S. Pat. No. 5,073,910 issued Dec. 17, 1991 to the present inventors discloses a complete data scanning path. That patent, which is assigned to the present assignee, is hereby incorporated by reference.
The criteria for data set completeness relative to scanning path in a 3D CT System are described in the paper by Bruce D. Smith entitled "Image Reconstruction from Cone-Beam Projections: Necessary and Sufficient Conditions and Reconstruction Methods", IEEE Transactions on Medical Imaging, Volume MI-4, No. 1, pages 14-25 (March 1985), hereby incorporated by reference.
Depending upon the particular object being viewed and the particular circumstances, it may be unnecessary to obtain a complete data scan under the criteria explained in the Smith article. Moreover, under some circumstances, it may be impossible or difficult to obtain a complete scan. There are occasions when data from an incomplete scan path is obtained under these and other circumstances such as when one initially thinks that the incomplete scan data will be sufficient for a particular purpose.
After obtaining incomplete scan path data relative to a particular object (as used herein, the object may be a workpiece or a medial patient or other thing which is being imaged), one may conclude that more accurate data is required in order to give an image satisfactory under the particular circumstances. For example, the incomplete scan path data may provide a satisfactory image over most of the object, but one may notice a particular portion of the object having an irregularity which must be imaged more accurately. Under such circumstances, one might simply start the imaging process over using a complete data scan path (in spite of the greater complexity of such a scan path) or at least using a scan path which will provide a more accurate image of the portion of the object associated with the irregularity. However, it may be difficult or inconvenient to perform a further scan on the object. For example, the object might be a work piece which had been imaged sufficiently satisfactorily that it was approved for use in a particular design and sent to another location for assembling the workpiece with other components of a machine. However, the engineers or others who are supervising assembly of the design may change the design such that the workpiece must now be checked for defects which would not have been a problem in the original design. In order to retest the particular workpiece under the more stringent criteria, the workpiece may have to be shipped back to the original testing location. This is a rather expensive and time-consuming step. Even under those circumstances where an object, such as a workpiece, is readily available for further scanning, further additional scanning may take some time to accomplish. Additionally, further scanning of the object may be convenient under certain circumstances, but may require one to repeat the original scan path in addition to adding lines or curves to the scan path.